Fuel control



March 7, 1967 c. F; sTEARNs FUEL CONTROL 4 Sheets-Sheet ll Filed Jari.15, 1964 C. F. STEARNS FUEL CONTROL 4 Sheets-Sheet 2 Filed Jan. l5, 1964C. F. STEARNS FUEL CONTROL Mach 7, 1967 4 Sheets-Sheet 3 1 Filed Jan.15, 1964 4 sheets-sheet 4 `FUEL CONTROL C. F. STEARNS March 7, 1967Filed Jan. 15, 1964 Patented Mar. 7, 1967 3,307,353 FUEL CONTROL CharlesF. Stearns, East Longmeadow, Mass., assigner to United AircraftCorporation, East Hartford, Conn., a corporation of Delaware Filed .lan.15, 1964, Ser. No. 337,904 Claims. (Cl. 60-39.28)

This invention relates to fuel controls and more particularly t-o fuelcontrols for controlling turbine types of power plants.

As is generally well known in the art, the fuel control is a combinationof metering devices and computing mechanism serving to control the flowof fuel to the engine in an amount commensurate with efiicient an-doptimum engine operation, yet assuring that a malfunction due to surge,

rich or lean flame out and overtemperature does not ensue.Overtemperature in this instance means that temperature which willadversely affect the components of the engine.

Basically, the metering system of the fuel control selects the rate offuel flow to be supplied to the engine burners in accordance with theamount of thrust (for pure jet) or horsepower (for turboprops and jetsdriving a variable load such as the rotor blades of a helicopter)demanded by the pilot, but subject to engine operating limitations asscheduled by the computing system as a result of its monitoring variousengine operational parameters. As is realized, the computing system ofthe fuel control senses and combines the various parameter to controlthe output of the metering section during all regimes of engineoperation.

In the heretofore known fuel controls for turbine power plants, such asthe type describe-d and claimed in Patent No. 2,822,666 granted to S. G.Best, engine scheduling is accomplished by relating all the operationalparameters being monitored by the fuel control in terms of scheduledratio of Wf/Pg, where Wf is fuel flow in pounds per hour, P3 is absolutecompressor discharge pressure. It is irnportant to understand here thatthis ratio of Wf/Pa is a value built into the fuel control by virtue ofcams, linkages, levers and the like which value represents the desiredengine operation. Hence, the ratio Wf/P3 may be considered to be ascheduled value, which for any given speed (r.p.m.) of the engines rotor(compressor and/ or turbine) defines all the operations of the engine.

In these heretofore fuel controls utilizing the Wf/Pg control parameter,the computing mechanism computes the various monitored engine operationparameters and converts these parameters in terms of this Wf/P3 ratio.

The fuel control also senses the actual Vcompressor discharge pressureand feeds this signal to the multiplying mechanism of the computingsystem which also receives the Wf/Pg ratio signal. Here these twosignals are multiplied for obtaining the product Wf of the two. This fsignal is then transmitted to the fuel metering devices for meteringsufficient amount of fuel to produce the desired thrust or horsepower.Basically, such a system describes the control philosophy for bothacceleration and steady state engine operation.

As is known in this art, the highest obtainable rate of acceleration isalways desirable in turbine types of power plants, and obviously, thisis accomplished by placing the metering -devices in their full flowposition. Owing to the fact that the component parts of the engine canonly tolerate a certain maximum temperature and that the compressor issubject to surge (a pressure pulsating condition that occurs at anyparticular given speed, at a given pressure ratio or given weight iiowof the compressor) the computing system must assure that theseconditions as well as rich and lean flame out do not ensue. Theseregimes of operation are computed by the fuel control computing systemwhich senses and combines various engine operating parameters.

The heretofore system such as the one noted in the Best patent, supra,utilizes a three-dimensional cam for computing the fuel rate necessaryto avoid the maximum temperature and surge. The three-dimensional cam isbest described as a cam having movement in an axial and rotationaldirection in response to selected engine operating conditions andoperates to give the temperature and surge limits. As mentioned above,the fuel control computing system must compute the value of thepreselected engine operational parameters in terms of Wf/P3 ratio sothat it will be compatible with the multiplication system. Thethree-dimensional cam serves this purpose for acceleration scheduling.This is accomplished by having the cam move in one direction in responseto compressor speed and having it move in another direction in responseto compressor inlet temperature. The cam follower responding to theradius of the cam develops a signal whose value corresponds to thedesired Wf/Ps value.

The cams profile comprises a plurality of curves superimposed thereondefining the engines operational characteristics in terms of Wf/P as afunction of inlet temperature of the compressor and compressor speed. Tomore fully appreciate the present invention, it is worthy to note thatthe engines surge and temperature characteristics can be defined for allengine operations in terms of Wf/P3 vs. engine speed. In such a plot itwill be appreciated that both surge and temperature form a family ofcurves where each curve of the family depends on the value of thecompressor inlet temperature. As is obvious to one skilled in this artthe turbine inlet temperature for any given speed of the compressor andvalue of Wf/P3 varies as a function of compressor inlet temperature.

As is well known by those familiar with the heretofore known fuelcontrols, surge and temperature limits are each represented by a familyof curves for the overall engine operation. It is also well known thatthe conventional three-dimensioned cam was the only known practicaldevice capable of defining a schedule representing two separate familiesof curves varying as a function of two independent input signals. Owingto this fact, it has been the heretofore practice to utilize athree-dimensional cam in this heretofore known fuel control.

It is a purpose of this invention to completely eliminate thethree-dimensional cam yet obtain accurate acceleration limit schedulingmechanism. We have found that this is made possible by utilizing WHZX/Paversus N2/02 or N/\/02 as the control parameters instead of theheretofore known Wf/P3 versus N control parameters; where N=the r.p.m.of engine rotating mechanism; 02=tem perature of the air upstream of thecompressor and x=an exponential value.

By utilizing the WfHzX/Ps vs. N2/02 as the control parameters, themaximum temperature limit schedule and surge limit schedule can each berepresented by a single function of the parameters. This is a completedeparture from the heretofore'knownfuel control philosophy since as wasdescribed above, the family of curves representing surge and temperaturebecome nonexisting. Owing to this fact the need of a three-dimensionalcam is completely eliminated.

It will be appreciated that the elimination of the threedimensional camis a major break-through in this art not only because it eliminates acostly item, but also because it allows independent and separatescheduling of the temperature and surge of the acceleration schedule.Hence, adjustment of either the surge or temperature limit schedule canbe accomplished independently. Y

Other features and advantages will be apparent from the specificationand claims and from the accompanying drawings which illustrate anembodiment of the invention.

FIG. 1 is a graphical illustration used to facilitate the derivation ofthe control parameter.

trol connected to a free-turbine type of gas turbineengine.

FIG. 3 is a schematic illustration of a fuel control mechanism utilizingthis invention.

FIG. 4 is a graphical illustration representing the operationalcharacteristics of a fuel control made in accordance with thisinvention.

FIG. 5 is a schematic of a hydraulic circuit illustrating a hydraulicmultiplication system.

FIG. 6 is a schematic of a hydraulic circuit illustrating an additionalmultiplication function of a hydraulic multiplication system.

FIG. 7 is a partial schematic showing a modication of FIG. 3, and

FIG. 8 is a block diagram illustrating the invention.

In order to appreciate this invention it is necessary to understand thatthe maximum temperature limit schedule of a gas turbine engine can berepresented by a single function of the parameters Wf02X/P3 and N 02 andthat the surge limit schedule can be represented by a sing-le functionof the parameters Wf/P3\/02 and N 02.A

Thus, the problem solved by this invention is to modify the Wf/P3parameter which changes with varying compressor inlet temperature toobtain constant turbine inlet temperature at any given corrected speed,i.e., N /\/02. It is within the scope of this invention if the terms(parameters) are computed in static or total values.

The following mathematical derivation shows how the enginecharacteristics have been computed to show that a single line functionof Wf02X/P3 versus corrected engine speed will hold actua-l turbineinlet temperature at a constant value for any value of engine inlettemperature. Small second order effects have been ignored in thederivation.

The following notations will be used for deriving the parameterWf62X/P3:

N \/2=Corrected engine speed in revolutions per minute WaV02/2=Correctedengine airflow in pounds/second I`=Turbine inlet ow numberP3/P2=Corrected compressor pressure ratio 17C=Compressor eciency ?11 P.4Pol4.7p.s.i.a.

d=Sign for derivative q--Constant Turbine inlet air flow is common-lyexpressed as a flow number F4, Where (l) F Wgr/Tl t" Arpt Using thecalculus this may be differentiated to the form (2) l1l4 =@e+l@ @-d f4iIi Wg 2 T4 P4 A4 Recognizing that the turbine inlet area is of iixedgeometry, and ignoring minor second order effects, this may bere-expressed as Y T 3/02=Corrected compressor outlet temperature in de-.l

grees (2) Rankine APb/PszBurner pressure loss A ATb/62=Corrected burnertemperature rise in degree Rankine f Y l Wf/ 2\/0 14=Corrected enginefuel ow in pounds/hour iyb=Combustion efficiency Wg=Wa+Wf=weight ofgas=weight of air-i-weight of fuel T4/02=Corrected inlet turbinetemperature in degrees Rankine 9P4/P5=Turbine pressure :1'atio" y j eqf=Fuel heating value in u y." i Y British' thermal units poundCp=Specic heat at constant pressure in British thermal units poundRankine (a) Phat a idr; Q

The general expression for ratio units in terms of other parameterswithin the engine is seoocpArnQ-a-nrb P3 f as) t -lbqf lfm VT4 Using thecalculus and ignoringgminor second order terms n this may bedifferentiated tothe form Y (55,. .0m/ y, Y,

Y P3 dATb l dT4 i/Vf F4 ATb 2 T4 Pa The burner temperature rise issimply ATb=T4T3 The rate of change cfburner temperature r'ise is then ndart" fri im *friert AT1, TATI, -Ti

engine compressor, the compressor temperature ratio is approximately (8)T3 P3 K- 1 This may be differentiated to the form Equations 7 and 9 maybe combined to the form,

Combining Equations 11 and 3 and collecting similar terms Equation l2then is a general expression for the rate of change of natio units interms of the rates of change of the turbine inlet flow number, turbineinlet temperature and engine inlet air flow. At any constant correctedengine speed (which is a condition of the original prob- `lernstatement) there is very little, if any, variation of turbine inlet ownumber due to changes in any other thermodynamic parameter, especiallywhen the turbine nozzles are choked, as they are in most engines overthe greatest part of their useful operating spectrum. Equation 12 maytherefore be simplified to To get the desired expression for the rate ofchange of ratio units in terms of the rate of change of turbine inlettemperature, it is necessary to establish the relationship between therates of change of air flow and turbine inlet temperature at constantcorrected speed. In the art, compressor performance is commonly depictedin a graphical chart form called a compressor map or compressorcharacteristic. FG. l will be recognized as a portion of such acompressor map, showing a portion of a single corrected speed line andtwo turbine inlet temperature lines, A and B. Value C is the change inengine air ow which will occur when going from turbine inletternperature value A -to value B at constant engine speed. Value D isthe change which would occur in compressor pressure ratio going fromturbine inlet temperature value A to value B at constant engine airflow. Obviously then 6 From Equation 3 and the problem statement it isobvious that 17) nia. i d Ti Wa= and that (18) dpa P3 dw, "l

T4= also dPa olWa P3- We dW P3 Wa P3 Where dWa -Wa dPs P3 CombiningEquations 20 and 13 and collecting terms,

Using the corrected quantity equivalents of the parameters, Equation 21may be expressed as d WL T4 T3 Pax/9L 02 02 K-l 1 1@ Wi AT., ATbzK-cdw.. 2 et Pavn @2 @2 W dpa-l-l 02 Equation 22 then expresses theinterrelationship that exists between the corrected ratio unit Wf/PBV'Hzand corrected turbine inlet temperature T 4/ 62 at any particularcorrected engine speed N/\/02 with minor second order terms beingignored.

The dilerential form, Equation 22 may now be expanded to to bedescribed4 hereinbelow.`

ATb

02 K4 1 AT., 2x11. dw, 1

W, 2 dps l T4 uff-1 m Trl-AT., 2rd,. dw..

WEI P3 P3 Letting the coefficient between lthe brackets be representedby the arbitrary symbol "x and using the calculus rto integrate Equation24 (25) Wwf/P3 :constant That is to say, if at any particular value ofcorrected engine speed a constant value of Wwf/P3 is scheduled to theengine, then actual turbine inlet temperature will not vary regardlessof the value of engine inlet temperature ratio 02. The actual value ofratio units scheduled to the engine will be As can be seen from Equation24 the x exponent contains terms related to burner temperature rise,compresso-r discharge temperature, compressor eiciency, and the slope ofthe compressor characteristic curves, all of which vary with correctedengine speed for any particular turbine inlet temperature. For manyengines it would be sufciently precise to let x be a constantirrespective of corrected engine speed; for others it would not, and xitself must also be v aried with corrected engine speed, as is I. yWf2X/P3 Referring now to FlG. 2 showing a gas turbine engine generallyillustrated by numeral itl` having a first section 12, wherein thecompressor is driven by turbine section 16. Since the turbine 16 isconnected to compressor 12 for driving the same, it is generallyreferred to as the gas generator turbine and the rotational speedthereof is hereinafter referred to as Ng. Turbine 1S illustrates a freeturbine which may be adapted to drive a variable load such as ahelicopter rotor, propeller, and the like. Since ftu-rbifne f8, is@mechanically disconnected from turbine 16 but is driven by thedischarging gases thereof and because itis free or has only anaerodynamic connection with the turbines in the first turbine section,it is generally referred to as the free turbine and the speed thereof ishereinafter referred to as Ni. Interposed between the compressor sectionand the turbine turbine section is the burner section generallyindicated by numeral 2t). Fuel is injected into the burner sectionthrough the fuel manifold 22. which is regulated by the fuel meteringsection lgenerally indicated by numeral 24 Basi-cally, the fuel meteringsystem of the fuel control serves to meter fuel to the engine in anamount commensurate with optimum engine operations while assuring thatmalfunctions due to surge, overtemperature or rich lor lean blow out donot ensue. Fuel is fed t-o fuel control 24 from reservoir 25 and thepressure thereof is increased by virtue of pump 2S through line 34D. The

fuel control also contains a drain manifold which dis-7,5

8 charges excessive fuel or ported fuel to drain via line 32.

The fuel control also contains a computing system which measures certainparameters, computes them in accordance with the control parametersindicated in the above for controlling the fuel metering system. Forthis purpose, compressor inlet temperature is sensed via line 34,compressor speed (Ng) is sensed via line 36, compressor dischargepressure is sensed via line 33 and frere turbine speed (Nf) is sensedvia line 40. It is to be understood that any suitable mechanism forsensing these various operating conditions of the engine is contemplatedwithin the scope of this invention. Power levers 42 and 44 are suitablysituated in the cabin of the aircraft and are available to the pilot forsetting the speed of the gas generator and the speed of the free turbinein a manner to be described hereinbelow, These control levers also serveto operate various other mechanically movable parts in the fuel control,also to be described hereinbelow. Thus, lever 42 rotates gear i6 whichrotates the connecting shaft and lever 44 rotates gear 48 which rotatesits connecting rotary shaft.

While the fuel control showing the prefer-red embodiment is illustratedin connection with a free turbine type of gas turbine engine, it is tobe understood that this invention has applications as will becomeobvious to one skilled in the art in connection with all types of gasturbine engines. As is known in this art, the free turbine can operateat a different speed than the gas igenerator. By virtue of this fact, itis often desirable to measure Nf as well as Ng. If, however, it isdesirable to utilize this invention in connection with a solid shaft ora coupled type of gas turbine engine, it would only be necessary tosense the speed of one of the rotating mechanisrns within the engine. Inthis event, the free turbine speed sensor which will be describedhereinbelow would not be utilized.

It is also to be understood and will be obvious to one skilled in theart that the terminology of power lever is not particularly limited tothe particular lever in the cockpit of the aircraft. Rather, it isintended to cover any linkage connecting the cockpit to the fuel controlwhether it be referred to as a 1go handle, power lever or throttle leveror the like. v

Now referring to FIG. 3 which is a schematic illustration Iof a fuel4control designated by numeral v2dy as having both a fuel meteringsystem and. acompruting system. Fuel is admitted to the throttle valvegenerally indicated by numeral 50 via inlet or supply line 30. Throttlevalve 5ft comprises spool 54 hav-ing a fluid reaction end 56 and acombined pressure and a spring reaction end 58. Adjustable spring 60acts against end 53 while uid admitted into chamber 62 acts against end56 and obviously the force generated by the pressure in chamber 6,2 andthe force generated by spring `6l) and the pressure acting on end 58will determine the position of spool S4. Fuel admitted to valve 5) fromlline 30 is 'metered by the metering edge L64 on spool 54 into line 22 byway of passage 6o and minimum' pressure and shutoff valve generallyindicated by numeral 68.

Minimum pressure and shutoff valve 68 comprises value member 70 urged inone direction by the spring 72. When the value of the pressure acting onthe underside or the left-hand end of valve member 70 is sufficient toovercome the force exerted by spring 72 as well as the pressure inchamber 74, the valve member is unseated allowing communication betweenline 66 and line 22.

As noted from FIG. 3, solenoid 76 normally urges ball valve 78 so thatline 80` interconnects chamber 74 with drain pressure P0 via clearancearound valve stem 82 so that drain pressure and the force of spring 72act on one end of valve member 70 opposing the force of the metered fueldischarging from line 66. Solenoid 76 may be actuated by the pilot bydepressing button 81 which conducts electrical current to actuate theplungers 82 and 84. Plunger 82 in this instance would move to the rightseator the strength of spring 99.

9 ing ball 78 against the drain line, interconnecting line 80 and line86. This serves to direct pressure upstream of throttle valve 50 behindvalve element 70 for urging this valve in the closed position forchanging to the emergency engine fuel ow.

So that the displacement of valve 50 and metering edge 64 is directlyproportional to the fuel passing therethrough, pressure regulator valvegenerally indicated by numeral 88 is employed. Valve 88 comprises aspool 90 mounted in cylinder 92 defining a pair of opposing chambers 94and 96. Disposed in chamber 94 is adjustable spring 99. Fluid upstreamand downstream of throttle valve 50 is admitted to chambers 94 and 96through line 9S, valve 100 and line 102 in one instance and line 104,ball valve 106 and lines 86 and 89 in the other instance. It will benoted from the drawing that valve 106 is seated on the left end blockingow between drain and line 104 while admitting pressure upstream of 50into chamber 96.

From the foregoing, it is apparent that valve member 90 is subjected toupstream and downstream pressure together with the force exerted byspring 99. This serves to position the metering portion 108 relative toorice 110. This orifice and metering element 103 serve to bleed pressurefluid upstream of valve 50 through line 112 into line 114 and eventuallyback to the reservoir, bypassing the throttle valve. Thus, it isapparent that the pressure drop across throttle valve 50 is maintainedat a constant value, which value is determined by the force In the eventa deviation of the desired pressure drop is evidenced, valve element 90will move relative to the metering edge 110 for opening or closing saidvalve and hence increasing or decreasing the rate of flow therethroughfor regulating the pressure drop across valve 50 to hold it constant.

It will be noted that valve 100 which is connected to lever 42 androtated thereby is normally in the open position. By rotating valve 100,fluid in line 102 is ported to drain via line 118. This decreases thepressure in chamber 94 and since the pressure acting on the other end isat agreater value, it permits the valve to move toward the right in thefull open position. This bypasses the fuel around the throttle valve anddirects the entire fuel back to drain. This completely starves theengine and prevents the fuel from increasing to a pressure whose valvewould be above the structural integrity of the components of the fuelcontrol. Overpressurization is occasioned by virtue of the fact that ifthe engine windmills by virtue of the air passing over the compressor,the fuel pump driven thereby would begin to overspeed and build up thepressure in line 30. What has just been described is the normal or mainfuel regulating means of the fuel control. This fuel control may alsoinclude an emergency system comprising shutoff valve 120, emergencythrottle valve 133, emergency pressure regulating valve 134 and valves106, 78 and 137 (note however, that valve 137 is only to shutoff theemergency system).

Of course, the emergency fuel control portion is only actuated in theevent that the normal fuel control becomes inoperative for one reason oranother. In that event, switch 81 is actuated for energizing solenoid 76for shifting the position of ball valves 78 and 106. This leads fluidfrom chamber 96 to drain via lines 104 and 83, leads fluid from cham-ber140 to drain via lines 143, 104 and 83 while chamber 74 is connectedwith high pressure via lines 86 and 80 for directing valve member 72 tothe closed position. Valve 133 is opened and is directly coupled tolever 42 for metering fluid to the engine through valve 120 and line 22.By virtue of the fact that the pressure underneath the valve element 146is greater than both the spring force and the pressure in chamber 140,it shifts to the open position allowing the communication between valve133 and line 22. Further, it will be appreciated that at this time Valve137 is directed to communicate line 148 to line 147 and allowingnormally closed emergency pressure regulating valve 134 to open.

10 This valve then serves to control the pressure drop across valve 133.Now that the fuel regulating system has been described, the next portionof the description will be directed to describing the computingmechanism of the fuel control.

As was mentioned above, the position of spool 54 of throttle valve 50 ispositioned by the pressure of the iiuid in chamber 62. This pressure ismade proportional to the desired amount of fuel which will operate theengine at a scheduled value determined by the fuel computing system. Thefuel computing system computes steady state and acceleration schedules-as will be described hereinbelow. In accomplishing this, levers 42 and44 are positioned in the desired position to4 develop the desired amountof thrust or horsepower necessary to propel the aircraft for its desiredoperation. Referring now to lever 42 which serves to select thepredetermined speedrsetting of the gas generator by Virtue of settingthe metering area defined by orifices and 132 of valve 134. This servesto set a desired area which will control the pressure in lines 136 and138 by bleeding fluid to drain. Ignoring for the moment the valvegenerally indicated by numeral 140, this pressure, in turn, establishesthe pressure in line 142. A pair of adjustable orices 144 and 146disposed in lines 136 and 138 respectively, serve to provide the idleand topping Ng limits by virtue of the fact that orice 130 is closedwhen lever 42 is set for topping and orifice 132 is closed when lever 42is set for idle.

From the foregoing it is apparent that valve 134 determines a portingarea for establishing a pressure in line 142 which is proportional tothe speed error of the gas generator compressor 12. In order toestablish a signal to compare the desired speed with actual speed, speedsensor general-ly indicated by numeral is employed and suitablyconnected to and driven by compressor 12 for rotating platform 152. Theplatform contains flyweights 154 and 156 which are pivotally connectedto the upstanding members 158 and 160. The arms of the ily- Weights bearagainst an end of pilot valve 160. When the iiyweights are disturbedfrom their vertical position resulting from the rotational movement ofplatform 152, they will either move inwardly or outwardly relative tothe rotational axis for positioning valve 160. This, in turn, metersHuid from line 162 to line 142 upstream of fixed restriction 166proportional to the square of the speed. It will be noted that thepressure in line 142 goes to chamber 163 via annual space 165 made atthelap lit between spool 161 and its cylinder and balances the flyweightforce which nulls the valve 161 at the right pressure.

It is apparent from the foregoing that the pressure in line 142downstream of orice 166 is a function of the actual speed of thecompressor and the desired speed generated by the position of lever 42.This value which is a speed error signal acts on the underside of valve168 for positioning the metering edge 170.

From the drawing it will be apparent that compressor discharge pressurefrom line 3S is admitted interally of bellows 172. The free end ofybellows 172 acts against pilot valve 174 which serves to regulatepressure in line 176 as a function of compressor discharge pressure.This is accomplished by metering fluid issuing from pressure supply line173 through passages in spool 174 and into chamber 175 via lines 178.Spool 174 is counterbalanced by the pressure in chamber 175 acting onits underside. Spool 174 translates with respect to the opening 180until the pressure in chamber 175 acting externally of bellows 172balances the pressure and the spring acting internally thereof. At thepoint of equilibrium, the pressure in line 176 is established at thepoint where it is proportional to compressor discharge pressure plus aconstant.

This pressure line 176 is then fed into pilot valve 168 which serves tometer it to line 180. By properly contouring metering edge 170, thispressure is then made a function of the desired fuel ow or Wf for steadystate operation. The fluid metered by metering edge is then fed intoselector valve 182 via branch line 184 where it is -admitted to chamber62 through line 186 when spool 188 is in the righthand position.

The position of lever 44 serves to generate a signal which will producea value for establishing the speed at which the free turbine is desiredto operate. This is accomplished by the rotation of valve 201 whichestablishes an area across orifices 202 and 204 venting fluid out ofchamber 206 formed in valve 140. This serves to control the pressuredrop across restrictor 208 for establishing the desired Nf. Adjustablerestrictions 210 and 212 downstream of orices 202 and 204 respectivelyestablish the minimum and maximum speed of the free turbine. Thepressure upstream of restrictor 208 is proportional to the square of thespeed of the free turbine which is established by pilot valve 210. Thisvalve operates substantially the same as valve 150 by coupling platform213 to the free turbine shown by line 40 in FIG. l so that flyweights214 responding to this speed positions pilot valve 211. This metersfluid issuing from servo supply line 216 into line 218 in such a manneras to make this pressure proportional to the free turbine speed squared.Hence, the pressure in chamber 206 is established as being thedifference between the actual speed generated by pilot valve 211 and thedesired speed established by the position of lever 44.

From the foregoing it is apparent that spool 220 is positioned as afunction of speed error for modifying the pressure in 4line 142. Thishas the effect of resetting the position of spool 168 and henceestablishing a new value for the setting of the gas generator or Ng.What has just been described is the steady state operation -asestablished by the computing mechanism of the fuel control. The nextportion of the description will describe the established accelerationsurge and overtemperature schedule ofthe computing mechanism.

As noted from FIG. 3, pressure proportional to compressor dischargepressure is admitted to temperature responsive valve generally indicated-by numeral 200 through line 222. T-he meter-ing edge 224 formed onspool 226 is made to vary as a function of the square root of 02.Compressor inlet temperature admitted to act externally of Ibellows 228through line 34 causes it to expand or contract for positioning spool226 as a function of compressor inlet temperature. By virture ofpositioning metering edge 224, the pressure drop across restrictor 230which bleeds fluid from line 232 to dr-ain is controlled and amultiplication is effectuated so that the pressure in line 232 `is madeproportional to compressor discharge press-ure and a function yof thesquare root of H2. This pressure is t-hen admitted to valve 234 whichAis positioned as a function of the speed Ng2/02 in the manner to bedescribed hereinbelow. Metering edge 238 of spool 240 is made to definean area which is a function of Wf/Pavz. The combination of this area andthe area established by fixed restri-ctor 242 serves to effect anothermultiplication so that -the pressure in line 244 is made proportional tothe desired fuel flow Wf.

The pressure in line 244 is then admitted into selector valve 2416 toact on the left end of spool 248 whi-ch in this instance is the desiredWf for establishing the surge limit lof the schedule. T-he pressureacting on the right end of spool 248 is the limit in terms of Wf for thetemperature limiting portion of the acceleration schedule. This isestablis-hed by metering lands 250 of spool 226 and 252 of spool 240 inthe manner described immediately below.

Fluid pressure proportional t-o compressor discharge pressure evidencedin line 176 is admitted -to the metering land 250 through line 256. Thismetering land is made a reciprocal function of 02 raised to the x powerwhich x power is established according to the m-athematical computationsnoted above. Hence, the pressure established in line 258 is amultiplication by virtue of metering edge 250` and fixed restriction260. The area defined by metering edge 250 varies as a function ofcompressor discharge pressure times a function of 1/02X. This fluid isthen admitted to valve 234 where metering edge 252 in conjunction withfixed restriction 262 serves to effect another multiplication. Since thearea defined by metering edge 252 is a function of Wf/P3 times 02X, thepressure in line 264 is proportional to the desired fuel fiow (Wf) forlimiting temperature.

This fiuid in line 264 is then admitted to selector valve 246 to act onthe right-hand end of spool 248. The difference between the two valuesof the pressures acting on either end of spool 248 determines theposition of spool 248 to either the left or right for admitting fluidinto line 265. Hence, the pressure in line 266 is either the desired Wfsurge or the desired Wf temperature depending on the position ofselector spool 248 of selector valve 246. This pressure is t-henadmitted to act on the left end of spool 188. As mentioned above,steady` state pressure proportional to the desired fuel fiow (Wf) actson the right-hand end. The difference between the two values willposition spool 188 to Ieither the left or right. The pressureproportional to the desired steady state Wt or the desired surge Wf ort-he desired overtemperature Wf is then admitted to chamber 62 forcontrolling the position of spool 54 of throttle valve 50.

Since it is desiralble to compute the terms of the parameters inabsolute values rather than gauge pressure values, absolute pressurecontrol 280 is employed. Fluid from the various computating valves andrestrictions whi-ch is eventually ported to drain is first admitted intochamber 282 via line 284 where it acts externally of bellows 286.Bellows 286 is evacuated and has its free end operatively connected toone end of spool 288. Since the Aother end of spool 288 iscounterbalanced by the pressure admitted thereto through line 290metering edge 292 thereof will assume a position to establish the drainpressure for establishing an absolute value datum line. Note that t-hepressure level is set equal to the spring constant established in valve171. Therefore, it will be appreciated that the datum line is equivalentto an absolute value so that the pressure control valves use a zeropressure datum rather than a gauge pressure datum.

As was pointed out in the albove, the exponential x for certain enginesmust be varied with the corrected engine speed. Mechanism foraccomplishing this is shown in FIG. 7. Basically the accelerationcomputing mechanism shown in FIG. 3 may be slightly modified to performthis function. This may be accomplished by adding a single land to spool226 of valve 200 and two lands to spool 240 and slightly rearranging thefluid connecting lines. Like numeral references correspond to like partsin the various drawings. Spool 240 like spool 240 of FIG. 3 ispositioned as a function of N2/0`2. This is achieved by developingpressure acting on the underside of spool 240 to a value proportional tothis term tby directing fluid from line 142 into line 255 by passing itthrough orifice 251 which modifies the pressure from line 142. Since thepressure upstream of restriction 166 is a function of the speed of thegovernor (see FIG. 3) and since the area defined by metering edge 251 isa function of temperature, the computing system serves to developpressure in line 255 and chamber 302 to be equivalent to a function ofN2/92.

In order to obtain the surge limit, pressure proportional to P3 is bledfrom line 176 through line 256 and restriction 306. The pressuredownstream of fixed restriction 306 is controlled by the metering edge308 formed on spool 240 which communicates line 256 to drain via lines310 and 312. Metering edge 308 is designed to cooperate with itsmetering port so that it defines a metering area which is a function ofWf/PSx/GZ. The cooperating between metering edge 308 and fixedrestriction 306 effectuates a multiplication for establishing thepressure intermediate thereof to be a function of Wf/\/2. The pressureacross fixed restriction 314 disposed in line its registering port toestalblish an area which is a function of the V02. This variablerestriction toget-her with the fixed restriction 314 cooperate in such amanner as to effectuate a still further multiplication of the Wf/\/2pressure to obtain a pressure which is proportional to the desired fuelflow Wf which value is the surge limit established by the computingsystem.

This pressure is then bled off branch line'32fl and di- -rected to valve246.

The next portion of the description will be concerned with the mechanismfor generating the temperature lim-it signal to valve 246. Fluid whosepressureis equivalent to compressor discharge pressure is bled from line176 through line 222 and restriction 322 and directed through line 326to metering edge 324 formed on spool 240. The area defined by meteringedge 324 cooperating with its registering port is made a function ofWf02X/P3. This metering edge position serves to communicate line 326with drain line 328 for controlling the pressure drop across restrictor322 for developing a pressure intermediate at a value proportional toWfzx. A pair of parallel disposed branch lines 330 and 332 lbleed fiuidfrom line 222 to metering edges 334 and 336, respectively, formed onspool 226. The areas of metering edge 334 and 336 cooperating with theirrespective ports Vary as a function of 1/02X where the x powercorresponds to different values, Thus for illustration purposes, assumethat the area of metering edge 334 is defined as l/02X1 and the area ofmetering edge 336 4is defined as l/02X2. These area, varying as afunction of compressor inlet temperature as sensed by bellows 228,communicate pressure in lines 330 and 332 with drain lines 338 and 340,respectively. This serves to control the pressure drops across fixedrestrictors 342 and 344, respectively. Consequently, the cooperationbetween variable restriction 334 and fixed restriction 342 effectuates amultiplcation f-or developing the pressure in line 330 which is afunction of fuel flow Wf corresponding to 02m. Likewise the pressure inline 332 established by variable restriction 336 and the fixedrestriction344 is made a function of the desired fuel flow in accordancewith ZX2. The pressure in lines 330 `and 332 are bled to metering edges343 and 346 formed so that they will define in cooperating wit-h theadjacent port an area which is a function of N2/02. These metering edgesserve to direct fluid from lines 348 an-d 350 into line 352 to give theproper temperature limit schedule for Nif/02, and therefore the properselection of 02X has ibeen made. v

From the foregoing, it is apparent that one end of the spool disposed invalve 246 is subjected to the scheduled overtempcrature limit and theother to the scheduled surge limit, and the least pressure value of the-two is transmitted to spool 188 of valve 182 shown in FIG. 3, where itis compared with the steady state signal.

OPERATION The operation of the fuel control can best be described byreferring to FIG. 4 which is a graphi-cal representation of theoperation of the engine. operation can be defined by the use of thecontrol parameter derived hereinabove where the acceleration schedule.Curve P represents the steady state engine operation and the plural-ityof curves R rep- It will be noted that the` resents an infinite numberof droop lines. Assuming that it is desirable to operate t-he engine atthe speed indicated fby numeral S which intersects the steady state linean-d the droop line and that the engine is just put in the on position.The pilot will cause levers 42 and 44 to rotate for establishing thiscondition which will automatically be achieved by the computingmechanism of the fuel control.

Immediately, fuel through line 30 will be fed to the minimum pressureand shutoff valve 68 and when its value overcomes the closing forcethereof it will tbe injected into line 22 from where it is delivered tothe burners. Simultaneously a signal generated by the steady statecontrol mechanism will cause the valve 50 to move towards wide openposition. This condition is represented by line T. Immediately uponintersecting line M the temperature computing mechanism begins to takeover the control of throttle valve 50 for limiting the fuel, which isaccomplished by valves 200 and 234 and speed sensor together with thecompressor discharge sensor 171.

The pressure established in line 176 is made proportional to compressordischarge pressure and is fed to line 256 to valve 200. Valve 200 whichis responsive to cornpressor inlet temperature positions metering edge250 metering fluid into line 258. By virtue of the area defined bymetering edge 250 with respect to the cooperating port which area is afunction of 02X and the fixed restriction 260 a multiplication isefiectuated so that the pressure in line 258 is made proportional toP3/02X. This pressure is then fed to valve 234 where metering edge 252in conjunction with the cooperating port defines an area which varies asa function of Wf62X/P3. It will be appreciated that the position of thisvalue is varied as a function of Ng2/02 which has been established bymetering edge 251 of spool 226. It will be appreciated that pressuregenerated @by pilot valve 161 is made proportional to Ngz, which, inturn, is fed to line 142 and admitted to metering edge 251. By virtue ofmetering edge 251 and fixed restriction 253` a division is effectuatedso that the pressure in line 255 is made proportional to N2/62 which, inturn, acts on the left end of spool 240.

By virtue of the relationship of metering edge 252 and fixed restriction262 another multiplication is effectuated so that the pressure in line264 which, in turn is then admitted on the right end of spool 248 ismade proportional to desired Wf. This causes a hydraulic signal to bedelivered through line 266 to act on the left end of spool 188 f-orcausing it to shift and in turn alter the pressure in chamber 62. OwingVto this pressure change, valve 50 is moved to a closing position forreducing the amount of fuel iiow to maintain the limit defined by lineM.

At the point where line M intersects line N the surge limit of theacceleration schedule will take over the control. This is eiiectuated bythe mechanism enumerated in connection with the temperature limit exceptdiffe-rent metering edges are employed. Here the fluid pressureproportional to compressor discharge pressure is admitted to meteringedge 224 via line 222. This metering edge in cooperation with thecooperating port defines an area which is a function of the V02. Thismeeting edge together with fixed restriction 230 effectuates amultiplication for establishing -a pressure in line 232 which is afunction of P3\/02. This pressure in turn is fed to metering edge 238which together with its cooperating port define an area which is afunction of Wf/P3\/02. This area togethcr with the area established byrestrictor 242 effectuates still anothermultiplication for establishingthe pressure in line 244 to be proportional to desired Wf. This fluid isthen admitted on the left end of spool 248 which, in turn, communicatesa pressure signal to line 266 for adjusting the position of spool 188.This, in turn, causes the fluid in chamber 62 to be modified to positionthrottle valve in accordance with curve N.

At the point where curve N intersects once again line mechanism.

From FIG. 4 it will be appreciated that fuel flow will be reduced untilthe control has established -an operation condition indicated by thepoint S. This is the controlling point established by the computingmechanism of the fuel control.

As has been mentioned above, the free turbine speed is set by the pilotby virtue of the position of lever 44. The gas generator producessuflicient power to maintain the selected Nf. Should the free turbinerun off speed, in the overspeed or underspeed direction, it will byvirtue of the speed sensing mechanism cause valve 140 to readjust. This[has the effect of resetting the speed of the gas generator an amountsufficient to change the power generated thereby for producing thenecessary energy of the working fluid passing through the engine fordriving the free turbine at the preselected speed.

THE HYDRAULIC MULTIPLICATION SYSTEM In order to establish the controlpressure, it was necessary in several instances to perform amultiplication. This multiplication was eifectuated by deiining an areaof a metering orifice to vary as a predetermined function. The followingdescription is illustrative of how the hydraulic multiplication iseifectuated.

The hydraulic multiplying device can be readily understood when thoughto-f as a device which produces a pressure which is a percentage of somesupply pressure. This device consists of liowing liuid through twoorifices in series as shown in FIG. 5.

If P supply represents one quantity and the y percent representsanother, then P2 represents the product of these two and amultiplication has been achieved; where P represents the pressure valueof the fluid.

As P supply varies, P2 will vary as )2% when the area of the oriiiceremains constant. As the area of orifice 1 or 2 is varied, thepercentage y will vary. It can be seen now that variations in thequantities to be multiplied can be controlled by either varying the Psupply or the areas.

A further multiplication can be accomplished by next using P2 as thesupply pressure of another series of orifices as shown cascaded in FIG.6.

This schematic represents the multiplication:

(l) P supply (percent A) :P2

P2 (percent B :Pz

P supply (percent A) (percent B)=Pz Now substitute: P3 (compressordischarge pressure) :P

supply, 2 (function of the compressor inlet temperature):

Percent A, P3\/t92 (function of the engine speed) =%B and Wf (enginefuel iiow) =Pz.

Therefore:

of P supply each other. Also in the interest of obtaining accuracy ofmultiplication, the iiow' 'through metering edges S224 and 250 will bemuch greater than that liowing through metering edges 238 and 252. v

While this invention was illustrated utilizing a hydraulic type ofcomputation system for a fuel control, it will be obvious to one skilledin the art that other mediums such as pnelumatics or electronics'and thelike may su'itably be adapted to practice this invention. What issignificant and contemplated by the scope of this invention is theteaching of anew control parameter Wf62X/P3 vs. Ng2/02 for controlling agas turbine engine. To this end and referring now more specifically -toFEG. 8, block 400 represents a suitable speed sensing device receivingan input speed signal and converting it to a function of the square ofthe speed (N2). This signal is then fed into block'402 representing adivider which also receives a second input signal 02 responsive to thetemperature at the inlet of the compressor. Here these two signals aredivided to produce an output signal equal to a function of N2/62. lThissignalthen becomes the input signal to the function generatorillustrated by block 404. It converts the signal to a value which isafunction of Wf/P3\/0 2. This signal is then multiplied in themultiplication box represented by numeral 406 which multiplies thisvalue by a signal equivalent to the function of P3\/02 which signal isestablished by block 408. Block 408 which represents a multiplicationdevice receives a signal which is a function of com pressor dischargepressure P3 and V02 and multiplies these two signals to generate theP3\/02 signal. Block 406 multiplying these two values, then develops aWf signal which is proportional to the amount of fuel established by thecomputation mechanisms. This signal is then fed to a gate type ofmechanism which senses another Wi signal developed by block 4l2. Gatemechanism 410 sensing these two input signals delivers the lower valueof thetwo to a second gate mechanism 414. Block 412 represents amultiplier which multiplies the two input signals'P3/92X and Wftlzx/PB.The Wf02X/P3 signal is *developed by function generator ilustrated byblock 416, as follows. Function generator 416 responding to the inputsignal N2/92 developed by divider 402 modifies it to a value equal to afunction of WfzX/Pa which is then transmitted to multiplier 4&2. Thesecond signal P3/62X which is multiplied by multiplier 4-1-2 isgenerated by function generator 42u 'by combining two input signals P3and @2X (generated by the function generator 422). The functiongenerator 422 receives tiwo input signals 02 and x (generated byfunction generator 424). The function generator 4.24 serves to convertN2/92 signal developed by divider 402 into the exponential x power.These two signals received by function generator 4122 in turnestablishes the signal 62X which is then combined with the P3 signal infunction generator 420 for producing the P3/02X. The multiplicationtaking place in the multiplier 412 develops the Wf signal which is thentransmitted to the gate mechanism 4:10. Gate mechanism 414 then comparesthe two Wf signals and transmits the least value of the two to block42?. which represents the metering valve for delivering suflicient fuelto the engine in accordance with the value determined by the computationsystem. The second Wf signal fed into the gate mechanism 414 isdeveloped by multiplier 430 which multiplies P3 and Wf/PB (developed byfunction generator 432). Function generator 4312 serves to combine asignal which is proportional tothe power lever setting and a signalwhich is proportional to speed for producing the Wf/Pg signal. Thissignal is then in turn transmitted to multiplier 4-30`which multipliesP3 and Wf/PS for developing the Wf signal. f

From vthe foregoing, it is apparent that function.' generator 43@ andmultiplier 4,30 develop the scheduled steady state Wf value forcontrolling the engine during the steady state operation. Multiplier412, function genserve to generate a Wf signal for limiting surge of theacceleration schedule.

For a clearer understanding of the mechanism represented by the variousblocks in FIG. 8, a graphical illlustration is shown therein toillustrate the desired function of each. lFor the sake of simplicity andconvenience and because the graphical illustrations are obvious to oneskilled in the art, a description of each is omitted from herein.

What has been described is a fuel control that assures accurate fuelmetering tor assure optimum engine performance and assuring thatmalfunctions due to surge, overtemperature and rich and lean flameolutdo not ensue. The fuel control monitors preselected engine operatingconditions and computes them to produce a signal (Wf) whose value isproportional to the desired fuel flow. The Wf signal is computed as afunction of and Ng2/02. By virtue of this parameter, the heretoforecustomary three-dimensional cam has been eliminated which has led to thesimplidication of the fuel control resulting in a less complex andrelatively less expensive control. Also, o-f relative importance is thefact that the temperature limit and the surge limit are eachindependently adjustable.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritor scope of this novel concept as defined by the following claims.

I claim:

1. A fuel control for a turbine type of power plant having a compressor,a burner and a turbine driven by the exhaust gases of said burner fordriving said compressor, a source of fuel under pressure, conduit meansinterconnecting said source and said burner, valve metering means insaid conduit means varying the flow therethrough, actuation means forvarying said metering means, computing means serving to control saidactuating means including a pair of serially connected valves eachhaving at least one metering land, at least one of said metering landsdening an area whose value varies as a function 1/0X, and the other ofsaid metering lands defining an area whose value varies as a function ofthe expression Wf/P@X Where Wf=rate of fuel P=compressor dischargepressure =compressor inlet temperature where T4=turbine inlettemperature ATb=burner temperature rise T3=compressor dischargetemperature K=ratio of specific heat of the working medium 1l=compressoreiciency dP P =slope of the speed line of the compressor in a graphplotting the pressure ratio versus the air flow of the compressor 2. Afuel control for a turbine type of power plant having a compressor, aburner and a turbine driven by the exhaust gases of said burner fordriving said compressor, a source of fuel under pressure and a drain,conduit means interconnecting said source and said burner, valvemetering means in said conduit means for regulating the flowtherethrough, -actuating means for varying said metering means, controlmeans including hydraulic conducting passage means, a pair of seriallyconnected valves each having at least one metering land in said passagemeans, one of said metering lands defining an area whose value varies asa function of l/HX, and the other of said metering lands defining anarea whose value varies as a function of the expression Wf/PX whereWf=rate of fuel P=compressor discharge pressure 0=compressor inlettemperature Tigra X41 2P 1 ya am AT1, 2K1, dWa Wa LFP" +1 where =slopeof the speed line of the compressor in a graph plotting the pressureratio versus the air flow of the compressor and a fluid connectioninterconnecting the passage means, at a point between the meteringlands, with said drain and a fixed restriction in said Huid connection.

3. A fuel control for controlling a turbine type of power plant having aturbine, a combustion section, a compressor driven by said turbine, asource of fuel under pressure, connection means interconnecting saidsource and said combustion section,

means for measuring compressor discharge pressure for producing a firsthydraulic signal,

means for measuring compressor inlet temperature for producing a secondhydraulic signal,

means for measuring compressor rotational speed for producing a thirdhydraulic signal,

means for combining said first, second and third signals to establish asurge and acceleration schedule according to the equation of WfX/P whereWf=weight of fuel flow in pounds per hour =compressor inlet temperatureP=compressor discharge pressure dWa Ws where T4=turbine inlettemperature ATb=burner temperature rise T 3=compressor dischargetemperature K=ratio of specific heat of the working medium v7=compressorefficiency CIW,

Ta=slope of the speed line of the compressor in a graph plotting thepressure ratio versus the P air fiow of the compressor means for varyingthe value of the exponent x as a function of the rotational speed of thecompressor.

4. A fuel control for a turbine type of power plant having a compressor,a burner and a turbine receiving the discharge gases of the burner fordriving the compressor,

responsive to compressor inlet temperature, a third valve responsive tocorrected compressor speed having a rst metering means seriallyconnected to said first metering means of said second valve and a secondmetering means serially connected to said second metering means of saidsecond valve each modifying said second and third hydraulic signals forproducing a fourth land fifth hydraulic signal, said first and secondmetering means of said third valve being parallelly disposed relative toeach other, and a selector valve connected to the first and secondmetering means of said third valve for selecting either the fourth orfifth hydraulic signal.

5. A fuel control as defined in claim 4 wherein said second hydraulicsignal is a function of the square root of 0 where zcompressor inlettemperature said third hydraulic signal is a function of 1/9X where xequals a thermodynamic value of the power plant working medium saidfourth hydraulic signal is a function 0f WVG/P Where W is weight of fuelin pounds per hour P is compressor discharge pressure and said fifthhydraulic signal is a function of WBX/ P.

References Cited by the Examiner UNITED STATES PATENTS 2,668,416 2/1954Lee 60--39.28 2,846,846 8/1958 Mock 60--39.28 2,933,130 4/1960 Wright etal. 60-3928 X 2,933,887 4/1960 Davies 60-39.28 X 3,025,670 3/1962 Russ60-39.28 3,032,986 5/1962 Wright 60-39.28 3,076,312 2/1963 Haigh60-39.28 3,098,356 7/1963 Joline 60-39.28 3,152,444 10/ 1964 Peczkowski60-39.28 X

JULIUS E. WEST, Primary Examiner.

4. A FUEL CONTROL FOR A TURBINE TYPE OF POWER PLANT HAVING A COMPRESSOR, A BURNER AND A TURBINE RECEIVING THE DISCHARGE GASES OF THE BURNER FOR DRIVING THE COMPRESSOR, MEANS FOR ESTABLISHING THE ACCELERATION SCHEDULE FOR THE ENGINE SO AS TO PREVENT SURGE AND OVERTEMPERATURE OF THE ENGINE INDEPENDENTLY OF EACH OTHER, SAID MEANS INCLUDING A FIRST VALVE RESPONSIVE TO COMPRESSOR DISCHARGE PRESSURE FOR ESTABLISHING A FIRST HYDRAULIC SIGNAL, A SECOND VALVE HAVING FIRST AND SECOND METERING MEANS LOCATED IN PARALLEL TO EACH OTHER BUT SERIALLY CONNECTED TO SAID FIRST VALVE FOR MODIFYING SAID FIRST HYDRAULIC SIGNAL AND PRODUCING A SECOND AND THIRD HYDRAULIC SIGNAL, SAID SECOND VALVE BEING RESPONSIVE TO COMPRESSOR INLET TEMPERATURE, A THIRD VALVE RESPONSIVE TO CORRECTED COMPRESSOR SPEED HAVING A FIRST METERING MEANS SERIALLY CONNECTED TO SAID FIRST METERING MEANS OF SAID SECOND VALVE AND A SECOND METERING MEANS SERIALLY CONNECTED TO SAID SECOND METERING MEANS OF SAID SECOND VALVE EACH MODIFYING SAID SECOND AND THIRD HYDRAULIC SIGNALS FOR PRODUCING A FOURTH AND FIFTH HYDRAULIC SIGNAL, SAID FIRST AND SECOND METERING MEANS OF SAID THIRD VALVE BEING PARALLELLY DISPOSED RELATIVE TO EACH OTHER, AND A SELECTOR VALVE CONNECTED TO THE FIRST AND SECOND METERING MEANS OF SAID THIRD VALVE FOR SELECTING EITHER THE FOURTH OR FIFTH HYDRAULIC SIGNAL. 